Thermal imaging 55 Cancri e.

55 Cancri (or Rho¹ Cancri) is a binary star system in the constellation of Cancer, 40 light years from Earth; it is naked eye visible. It comprises two stars, 55 Cancri A, a Sun-like G-type star with 95% of the Sun's mass, 1.15 times its radius (6.4 times its volume) and an effective temperature of 5373 K (compared to 5778 K for our Sun), which is orbited by 55 Cancri B, a smaller, cooler Red Dwarf star, at a distance of 1000 AU (i.e 1000 times the distance at which the Earth orbits the Sun, or 33.3 times the distance at which Neptune orbits). The system also has at least 5 planets.

Finding 55 Cancri within the constellation Cancer. University of Illinois.

The first of these, 55 Cancri b (or 55 Cancri Ab to distinguish it from 55 Cancri B) was detected in April 1996, by scientists at the Lick Observatory in California, who were able to detect the wobble of 55 Cancri A caused by the gravity of the planet as it orbits the star. 55 Cancri b orbits 55 Cancri A at a distance of 0.115 AU (11.6% of the distance at which the Earth orbits the Sun, less than half the orbital distance of Mercury) every 14.6 days. It is thought to have a mass slightly over that of Jupiter.

Two further planets were discovered in June 2002, by the same team, using a refinement of the technique that enabled subtler influences on the star's movement to be detected. 55 Cancri c has a mass 0.169 times that of Jupiter (54 times that of Earth), and orbits 55 Cancri A every 44.3 days at a distance of 0.24 AU (two thirds of Mercury's orbit). 55 Cancri d has a mass 3.8 times that of Jupiter and orbits 55 Cancri A at a distance of 5.77 AU (comparable to 5.2 AU for Jupiter) every 5218 days (14.3 Earth years).

In August 2004 a fourth planet was discovered, using the Hobby-Eberly Telescope in Texas, again by its gravitational influence on the star. 55 Cancri e has a mass 7.81 times that of Earth and orbits 55 Cancri A at a distance of 0.0156 AU, every 18 hours.

In April 2005 a fifth planet, 55 Cancri f, was discovered using further refinements on the gravitational wobble of 55 Cancri A, and data from the Lick and Keck Observatories. This planet orbits at 0.78 AU every 260 days, placing it firmly within 55 Cancri's habitable zone (the zone in which a reasonably Earthlike planet might host liquid water, and therefore potentially life), though with a mass of at least 0.144 that of Jupiter (at least half that of Saturn) 55 Cancri f is unlikely to do so.

Diagram showing the relative orbits of the known planets of 55 Cancri. Orbits of the planets of our Solar system (grey) shown for comparison. The Visual Exoplanet Catalogue.

Uniquely among these planets, 55 Cancri e transits (passes in front of) its star, when seen from Earth, giving it greater potential for further study. In a paper published on the arXiv database at Cornell University Library on 8 May 2012, and accepted for publication in The Astrophysical Journal Letters, a team of scientists led by Brice-Oliver Demory of the Department of Earth, Atmospheric and Planetary Sciences at the Massachusetts Institute of Technology preset the result of a study of 55 Cancri e's infrared emissions during a series of transits of 55 Cancri A, made using the Spitzer Space Telescope.

They obtained an estimated temperature of 2360 K for 55 Cancri e, which is hot, but cooler than expected for a planet this close to a star. This suggests it has a low Bond albedo; i.e. it reflects less of the infrared radiation that falls onto it back into space than would be expected. This could be caused by a temperature inversion in the atmosphere, i.e. a warmer layer of gasses trapped below a cooler one, but this would need to be very large (over 500 K) which is improbable. Alternatively it could imply that the planet is rocky with little atmosphere (Mercury and the Moon both have low Bond albedos), which would be surprising for a planet of this mass, but could be explained by its proximity to 55 Cancri A, which could have evaporated its atmosphere away (the 55 Cancri system is 7.4-8.7 billion years old).

An artist's impression of 55 Cancri e. NASA/JPL.

See also Looking for HD 97658b, Planets discovered orbiting the ancient, second generation star HIP 11952, The atmosphere of WASP-24b, Cooking the planets if CoRoT-7 and Exoplanets on Sciency Thoughts YouTube.

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Cooking the planets if CoRoT-7.

The CoRoT-7 stellar system was discovered in 2009 by the European Space Agency's COROT space probe. The system is roughly 489 light years away in the constellation of Monoceros, and comprises of a G-type star (CoRoT-7A)with 91% of the Sun's mass surrounded by at least two planets. The first of these, CoRoT-7b, orbits at a distance of 0.017 AU (i.e. 1.7% of the distance between the Earth and the Sun) every 20.5 hours and has a mass of about 7.4 times that of the Earth. The second planet, CoRoT-7c, orbits CoRoT-7A at a distance of 0.046 AU (4.6% of Earth's orbital distance) every 3.7 days and has a mass about 15 times that of the Earth. A possible third planet, CoRoT-7d, may orbit at 0.08 AU, and have a mass over 16 times that of the Earth's.

An artist's impression of CoRoT-7b. Catalano Fabien.

In a paper published on 19 March 2012 on the online arXiv database at Cornell University Library and accepted for publication in the journal Astronomy and Astrophysics, a team of scientists lead by Katja Poppenhaeger of the Hamburger Sternwarte, carry out an examination of the effects of being this close to the star on the planets of the CoRoT-7 system, based upon new observations by the European Space Agency's XMM Newton X-ray space telescope.

Poppenhaeger et al. calculate that CoRoT-7b is slowly being evaporated by the heat of it's star, losing 130 000 tonnes of mass every second. The CoRoT-7 system is thought to be about 1.5 billion years old, so if CoRoT-7b has not moved radically within the system during this time, and its composition has not changed greatly either, then it is likely to have lost between four and ten times the mass of the Earth during its history.

They also estimated that CoRoT-7c would be losing mass due to evaporation as well, but at roughly 10% of the rate for CoRoT-7B (i.e. about 13 000 tonnes of mass per second). However unlike CoRoT-7b, CoRoT-7c does transit the star, so that its radius and density cannot be calculated, making calculations on its makeup, and therefore how it will react to irradiation, highly speculative.

See also Wasp-12b; slowly boiling away... KOI-13b, a big, hot planet not a Brown Dwarf, Wasp-19b: a Highly Irradiated Hot Jupiter, A planet disintegrating in the heat of its own sun and Exoplanets on Sciency Thoughts YouTube.